The replacement of human blood platelets and leukocytes by transfusion of allogeneic cells collected from volunteer donors provides an important approach to the therapy of pancytopenia resulting from depression of bone marrow function. Efforts to improve procedures for collection of large numbers of blood platelets, granulocytes, lymphocytes and bone marrow stem cells will be investigated. These include the study of biomechanical equipment, such as the continuous flow centrifugal blood cell separator. This instrument can separate blood into 3 components and offers the potential for component blood donation. Additionally, studies to prolong the in vitro storage of such cells could greatly improve the efficiency and effectiveness of replacement transfusion therapy. Identification of the immunologically significant antigens for predicting compatibility between donor and recipient could result in greater safety and effectiveness of transfusions. The role of platelet and leukocyte transfusions in the supportive therapy of patients with malignant diseases undergoing intensive therapy will be investigated. Evaluation of the physiology of such transfused cells should assist in developing indications for the use of such treatment, as well as objective criteria for evaluating response. The potential for myeloid allogeneic homografts resulting from leukocyte transfusion offers an approach to prolonging the duration of support for myelosuppression. The transfer of immunological information from transfusion of lymphocytes requires efforts to prevent reaction of grafted cells against the recipient and offers the potential for potential for immunotherapeutic approaches to cancer therapy.